`BlockWriteConfig`: option for write failed

[Tom-Willemsen]

As a user I would like options about what to do if the write to a BlockRw fails.

I might want to either error, or ignore it and carry on measuring anyway (wherever the value happened to end up).

Acceptance criteria

• BlockWriteConfig has an option for what to do if the write "failed" (e.g. hit it's timeout without getting within tolerance or similar)

Notes

Implement this by wrapping the existing code in set() in BlockRw in a try-except, which reraises by default when e.g. timeout is hit, but can optionally swallow the exception if explicitly asked to ignore failed writes.

Default behaviour should be that failed writes cause an error.